1. Field of the Invention
The present invention relates to a semiconductor device having a circuit that is composed of a thin film transistor (hereinafter referred to as TFT) and a method of manufacturing the semiconductor device. An example of the semiconductor device is electronic device having as one of its parts an electro-optical device, typically, liquid crystal display panel. Specifically, the present invention relates to a liquid crystal display device that uses a ferroelectric liquid crystal.
In this specification, the term semiconductor device refers to devices that utilize semiconductor characteristics to function, and electro-optical devices, semiconductor circuits, and electronic device are all regarded as semiconductor devices.
2. Description of the Related Art
In conventional liquid crystal panels, a liquid crystal is sandwiched between two substrates which are arranged in parallel to each other. The surfaces of the two substrates that are in contact with the liquid crystal often receive some alignment treatment in order to orientate liquid crystal molecules in a certain direction.
Examples of known alignment treatment include rubbing in which a thin film called an alignment layer is formed on a substrate in advance and then rubbed in one direction with cloth, and optical alignment in which an alignment layer is irradiated with polarized ultraviolet light to make the alignment layer anisotropic.
Among numerous liquid crystal materials, there are enumerated as thermotropic liquid crystal materials nematic liquid crystals whose molecules are generally aligned in one direction by alignment treatment; smectic liquid crystals whose molecule groups face the same direction and are stacked to form layers: and cholesteric liquid crystals which have a twist (helical) structure in addition to the same characteristic of nematic liquid crystals.
For smectic liquid crystals, which are represented by antiferroelectric liquid crystals and ferroelectric liquid crystals, it is difficult to achieve uniform alignment solely by the above alignment treatments.
Given below are reasons why achieving uniform alignment by conventional alignment treatment is difficult in ferroelectric liquid crystals.
FIGS. 7A and 7B show a ferroelectric liquid crystal interposed between a pair of substrates which have received conventional alignment treatment. A liquid crystal alignment direction 101 immediately after the liquid crystal is injected generally matches a direction 102 intended by the alignment treatment. At a closer look, however, the liquid crystal alignment direction 101 consists of two different alignment directions, 101A and 101B, which crisscross the substrate plane. This is because on one hand ferroelectric liquid crystal molecules naturally align in a direction close to the direction 102 intended by the alignment treatment but on the other hand the ferroelectric liquid crystal has two spontaneous polarization directions 103: a first substrate side direction 103A and a second substrate side direction 103B, to create different alignment states. When the mixed alignment states are observed under a polarizing microscope, a mosaic-like alignment pattern called a domain 104 and a zigzag dividing line are found. Therefore, high quality display cannot be obtained.
The presence of the domain 104 and the dividing line is recognized by a viewer as light leakage and an uneven image, namely, defective liquid crystal alignment, and greatly affects the quality of a displayed image. To give the liquid crystal display device a decent display quality, at least the domain 104 has to be removed by integrating the two alignment directions of the liquid crystal molecules into one.
There are several methods to achieve this. In one of those methods, the first step is to heat a pair of substrates that sandwich a ferroelectric liquid crystal uniformly throughout the substrate plane for phase transition of the liquid crystal. As a result, the liquid crystal is changed into the isotropic phase (I phase) or the chiral nematic phase (N* phase). Thereafter, the temperature is gradually lowered while applying direct current electric field to the pair of substrates in the direction of perpendicular to the substrates. When the temperature is dropped past the phase transition point, the ferroelectric liquid crystal returns from the isotropic phase (I phase) or the chiral nematic phase (N* phase) to its initial phase such as the chiral smectic C phase (SmC* phase). If electric field is applied to the liquid crystal at this point, because of the nature of the ferroelectric liquid crystal that directs its spontaneous polarization in one direction in accordance with the direction of the electric field, one of two different alignment directions becomes stable and resultantly, the alignment directions of the liquid crystal molecules are integrated into one. The electric field is turned zero after the phase transition into the chiral smectic C phase (SmC* phase) is induced by temperature drop and completed. This liquid crystal alignment method is called monostabilization.
Theoretically, a ferroelectric liquid crystal should be orientated uniformly by the above monostabilization method. In practice, however, defects accompanied with monostabilization treatment are often found that some regions deviate from the intended alignment. In some cases, defects accompanied with monostabilization treatment is caused due to influence of other components of the panel, for example, seal and a spacer. In other cases, defects accompanied with monostabilization treatment is caused by treatment conditions set inappropriately in injecting the liquid crystal and in monostabilizing the liquid crystal.
As shown in FIG. 8, defects 201 accompanied with monostabilization treatment is found particularly frequently in an area 202 near the inlet of the panel. The area 202 near the inlet serves as paths 203 through which every liquid crystal molecule enters the panel upon injection of the liquid crystal. Commonly, the inlet is made as narrow as possible for the sake of panel reliability and resultantly, injected liquid crystal molecules concentrate in the area 202 near the inlet. The amount of liquid crystal that passed the area 202 near the inlet is by far large compared with the rest of the panel.
The injected liquid crystal causes friction against the alignment layer along the liquid crystal molecule paths 203. If the frictional force is large, the effect that presents is similar to rubbing and could change the alignment of the liquid crystal rubbing against the alignment layer. The liquid crystal alignment direction set by the frictional force that is generated along the liquid crystal molecule paths 203 does not always match an alignment treatment direction 204 intended by a panel designer. When the two does not match, the panel locally suffers the defects 201 accompanied with monostabilization treatment.
Defective monostabilization could be avoided by optimizing liquid crystal injection conditions, alignment treatment conditions, and monostabilization conditions. In practice, however, optimizing these conditions is laborious and it seems that the merest margin is allowed for each condition. The inventors of the present invention have conducted experiments but have not been successful in finding out parameters which truly save a liquid crystal panel from the defects accompanied with monostabilization treatment.
Another method that may counter defects accompanied with monostabilization treatment is to prevent it from taking place in a display area by adjusting the position of a liquid crystal inlet of a liquid crystal panel such that the liquid crystal inlet is as far away from the display area as possible. However, putting a distance between the liquid crystal inlet and the display area creates an idle space within the panel. In addition, this method is not helpful in panel size reduction, which is a common way to lower the manufacture cost of the liquid crystal panel and the cost of panel's peripheral devices.